The present invention relates to sheet feeding apparatus and in particular to vacuum sheet separating and feeding apparatus. A specific embodiment is directed to a sheet stacking tray for use with a bottom vacuum corrugating feeding apparatus.
With the advent of high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, the need for a document handler to feed documents to the copy platen of the machine in a rapid, dependable manner was recognized to enable full utilization of the reproduction machine's potential copy output. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the originals and generate minimum machine shutdowns due to uncorrectable misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of separators have been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member, if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document. With single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
In addition, a typical vacuum separating and feeding system is that described in Ser. No. 081,594 entitled "Sheet Separator" to Hamlin, now U.S. Pat. No. 4,305,576 wherein a plurality of friction belts is arranged to run over a vacuum plenum placed at the bottom of a sheet supply tray which has a "U" shaped pocket formed in it. The pocket serves to provide space for the bottom sheet to be captured by the vacuum feed belt assembly, to provide an air seal between the bottom document and the edges of the pocket and to provide a high pressure seal between the bottom sheet and the remainder of the stack. This high pressure seal is achieved by supporting a major portion of the stack weight on the edge regions of the pocket. This seal serves to convert the velocity energy of the air knife flow into a lifting pressure over the pocket area to levitate the remainder of the stack of sheets. This configuration has been used on a commercial scale in the Xerox 5600 machine and while it has been highly successful in operation, certain aspects can be improved. In particular the operating window for air knife pressure and stack weight is relatively low which when exceeded causes an unstable pocket to exist. This is manifested by the second sheet vibrating independent of the rest of the stack in a manner referred to as "flutter" and is caused by an aerodynamic instability due to a very low angle of attack of the air knife relative to the stack together with the springness of the sheet. As a result there is a dynamic bouncing of sheet two on sheet one and while it is not coincident with sheet one, sheet two will tend to shingle out of the stack with sheet one resulting in a multifeed. In addition, since the bottommost sheet and sheet two are in the same plane and with sheet two and the remainder of the stack resting on the bottommost sheet, difficulty in separating the bottommost sheet from sheet two can be encountered. This is because there may be some frictional bonding between the bottommost sheet and sheet two.